Subantarctic mode water (SAMW) is a key water mass for the transport of nutrients, oxygen, and anthropogenic carbon into the ocean interior. However, a lack of biogeochemical observations of SAMW properties during wintertime formation precluded their detailed characterization. Here we characterize for the first time SAMW properties across their entire wintertime formation regions based primarily on biogeochemical profiling floats. Observations show that the SAMW properties differ between the two main formation regions in the Pacific and Indian sectors of the Southern Ocean. SAMW formed in the Pacific is colder, fresher, and higher in oxygen, nitrate, and dissolved inorganic carbon (DIC) than its Indian Ocean counterpart. The relationship between potential density and biogeochemical water properties is nearly identical between the two formation regions; property differences thus predominantly reflect the difference in mean densities of SAMW formed in each region. SAMW is undersaturated in oxygen during formation, which will impact calculations of derived quantities that assume preformed oxygen saturation. SAMW is at or above atmospheric pCO2 during wintertime and therefore not a direct sink of contemporary carbon dioxide during the formation period. Results from the Biogeochemical Southern Ocean State Estimate suggest anti-correlated interannual variability of DIC, nitrate, and oxygen between the central and southeastern Pacific formation regions similar to previously established patterns in mixed layer physical properties. This indicates that the mean properties of SAMW will vary depending on which sub-region has a stronger formation rate, which is in turn linked to the Southern Annual Mode and the El-Niño Southern Oscillation.

Key Points

Subantarctic mode water (SAMW) biogeochemical formation properties are a function of the density of newly formed water

Newly formed SAMW is undersaturated in oxygen due to opposing effects from cooling (solubility) and entrainment, and air-sea injection

SAMW is near or above atmospheric pCO2 during formation and therefore not a strong direct sink of contemporary carbon dioxide

Plain Language Summary

In the Southern Ocean, north of the Antarctic Circumpolar Current, wintertime surface ocean heat loss cools the water, increasing its density and forming thick layers of well mixed water that enter the ocean. This water, called Subantarctic Mode Water (SAMW), represents an important pathway for anthropogenic carbon, nutrients and oxygen into the ocean interior. In this study we used new wintertime observations from profiling robots equipped with sensors that measure oxygen, nitrate, and pH in the top 2,000 m to determine important initial properties of SAMW for the first time. We find that the SAMW properties differ between the Pacific and Indian formation regions and are related to the densities of SAMW formed in each basin. These properties indicate that it is unlikely for SAMW to take up present-day carbon dioxide from the atmosphere during formation, though it may still absorb anthropogenic carbon. We investigated how these properties varied year-to-year using an ocean model linked to observations, finding connections between changes in the biogeochemical properties and physical processes as well as large-scale climate variability. These results will provide valuable constraints on interpretation of subsurface ocean measurements and model studies investigating the role of these waters in the global carbon cycle.